Set `overflow-y` to `scroll` to prevent reflow

Setting `overflow-y: scroll` to prevent reflow as soon as content
comes in. Adding it to `body` as there might be side-effects when
it's set on both, `html` and `body` and latter seems to be the
place where volunteers or third parties would have set it already.

Bug: T204084
Change-Id: Id9ee2e85225f969b6a286628b70fb7a4e447040c

components/common.less

@@ -20,6 +20,7 @@ body {
 
 body {
 	background-color: @background-color-secondary;
+	overflow-y: scroll;
 }
 
 /* Content */